This invention relates to drive systems for variable speed alternating current (a-c) electric motors, and more particularly it relates to a "cycle-skipping" kind of motor speed control system.
Cycle-skipping speed control systems for single-phase a-c electric motors are well known in the art. See, for example, the paper "Low-Cost Electronic Speed Controller for Small Induction Motors" by W. Matley, K. B. Clarke, and P. Marshall presented at the Second International Conference on Electrical Variable-Speed Drives in London, England on Sept. 25-27, 1979 (IEE Conf. Publ. No. 179, pages 42-45). In a typical system of this kind a controllable bidirectional solid state switch (e.g., a pair of silicon controlled rectifiers or thyristors which are interconnected in inverse-parallel relationship with one another) is connected between the motor and a single-phase source of alternating voltage of constant frequency, and the switch is suitably controlled (when reduced speed is desired) so that selected half cycles of the source voltage are omitted from the voltage applied to the motor. Consequently the fundamental frequency of the output voltage is a predetermined fraction of the fundamental frequency of the source voltage, and the running speed of the motor will be correspondingly reduced compared to full speed. According to Matley et al, the source frequency can be reduced in steps of odd integers. U.S. Pat. No. 4,176,306 discloses a 3-phase application of such a system, wherein the maximum output frequency at low speed is one-fifth of the source frequency.
According to prior art U.S. Pat. No. 3,908,158, substantially constant motor torque can be maintained during reduced speed operation of a single-phase cycle-skipping system by retarding the "firing angle" of the bidirectional switch, whereby less than all of each non-skipped half cycle of source voltage is applied to the motor at the reduced gating frequency. This desirably reduces the amplitude of the fundamental component of the reduced-frequency output voltage. To control the frequency at which the switch is gated, U.S. Pat. No. 3,908,158 discloses a countdown circuit which requires that the source voltage have a constant frequency for proper operation.
Another prior art speed control system for driving an a-c motor at reduced speed is known as a "cycloconverter". In a typical polyphase cycloconverter, three controllable bidirectional switches are respectively connected between each terminal of the motor armature and the alternative phases of a 3-phase source of alternating voltage, and the switches are suitably controlled so that the fundamental frequency of the output voltage is desirably reduced compared to the frequency of the source voltage. A cycloconverter can be used with either a constant frequency source or a variable frequency source. However, its control circuits are somewhat complex, and it usually requires a relatively large frequency reduction for proper operation.